It is known that pumping of fluids containing gases, with higher gas contents, is unsuccessful without a gas discharge system because the gas concentrates around the center of the pump rotor, forming a bubble which grows thus tending to clog the entire inlet opening of the pump. This results in a considerable decrease of the output, increased vibration of the equipment, and in the worst case, malfunctioning of the pump. This problem has been experienced in a very severe form with, for example, centrifugal pumps.
These problems have been attempted to be solved in many different ways by discharging the gas bubble from the pump. In the equipment presently known and used, degasification is effected by either drawing gas through a pipe being disposed in the middle of the inlet opening of the pump and extending to the hub of the impeller, by drawing gas through a hollow shaft of the impeller, or by providing the impeller with one or more perforations through which the gas is drawn to the back side of the impeller and away.
The known solutions function satisfactorily if the fluid is neat or pure. Problems, however, arise when the fluid contains foreign matter such as fibers, threads and the like. In such a case, the contaminants tend to clog the gas discharge duct of the pump, the staying open of which is a matter of necessity for the proper operation of the pump. Several different arrangements are known by means of which it has been tried to eliminate or minimize the disadvantages or risks caused by contaminants. The simplest arrangement is a gas discharge duct which is so wide that clogging will not occur. Other methods used are, for example, arrangements with various types of vanes or vaned rotors on the back side of the impeller. A commonly used method has been to provide the immediate back surface of the impeller with radial vanes which are intended for pumping the gas containing suspension with its contaminants so that the suspension with the gas entrained therein is directed through the gas discharge openings of the impeller to the outer periphery of the impeller and through its clearance back to the liquid flow. In some cases, a similar type arrangement has been provided on the back side of the impeller by means of a vaned rotor mounted on the shaft of the impeller. Said vaned rotor rotates in a separate chamber thereby separating the liquid which has been carried with the gas to the outer periphery of the chamber, while the gas can be further drawn to the inner periphery. The liquid accumulated on the outer periphery of the chamber is directed, together with the contaminants contained therein, through a separate duct to either the inlet side or the outlet side of the pump.
All of the above described devices operate satisfactorily if the amount of contaminants or foreign matter being carried with the liquid is limited. It is also possible to adjust the devices to operate relatively reliably with liquids that contain greater amounts of solids, e.g. with fiber suspensions in the pulp industry. In that case, however, a compromise must be reached between pumping a suspension having solids content and the necessity of discharging gas because it is of utmost importance to ensure that no fibers are conveyed to the gas discharge duct of the pump. Thus, a fiber suspension containing gas is returned back to the flow. On the other hand, it is known that the gas contained in the fiber suspension is undesired in the stock preparation process and should be avoided as far as possible. Therefore, existing advantages of these known pumps are wasted by feeding the gas that has already been separated back to the stock circulation. It is also wasting of stock if, on the other hand, all stock conveyed along with the gas were separated from the stock circulation by discharging the stock as a secondary flow of the pump.